1. Field of the Invention
The present invention generally relates to exhaust gas cleaning apparatuses for internal combustion engines. More specifically, the present invention relates to an exhaust gas cleaning apparatus that regenerates an exhaust gas cleaning device by changing the excess air ratio.
2. Background Information
In internal combustion engines such as diesel engines, an exhaust gas recirculating system (EGR system) is widely used wherein a part of the exhaust gas is recirculated to lower the combustion temperature in order to reduce discharge of nitrogen oxide (NOx). A NOx trapping catalytic converter traps NOx in the exhaust gas when an air-fuel ratio in the exhaust gas is in a lean range and purifies (releases) the trapped NOx when the air-fuel ratio is in a rich range. The NOx deposited in the NOx trapping catalytic converter is typically purified when the amount of adsorbed and held NOx reaches a prescribed upper limit value.
One example of an internal combustion engine having a NOx trapping catalytic converter provided in an exhaust system for cleaning NOx discharged from the engine is disclosed in U.S. Pat. No. 5,732,554 (also see, Japanese Laid-Open Patent Publication No. 08-218920). The NOx trapping catalytic converter described in that document is configured to adsorb and hold NOx contained in the exhaust gas when the excess air ratio is lean, and to desorb and deoxidize the adsorbed NOx when the excess air ratio is rich. These types of NOx trapping catalytic converters are generally applied to internal combustion engines that are normally operated at a lean excess air ratio. When the amount of adsorbed and held NOx reaches a prescribed upper limit value, the NOx trapping catalytic converter can no longer adsorb and hold more NOx. Consequently, the amount of held NOx is estimated and the NOx trapping catalytic converter is regenerated.
When the NOx trapping catalytic converter is regenerated, an intake air throttle valve or the like is used to reduce the amount of intake air into the engine to reduce the excess air ratio to a richer target value. Simultaneously, the fuel injection amount is increased such that the excess air ratio converges on the richer target value.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved internal combustion engine exhaust gas cleaning apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.